Unified and mixed formulation of the 8-node hexahedral elements by assumed strain method

In this paper, a class of 'assumed strain' mixed finite element methods based on the Hu-Washizu variational principle is presented. Special care is taken to avoid hourglass modes and shear locking as well as volumetric locking. A unified framework for the 8-node hexahedral solid and thermal as well as thermomechanical coupling elements with uniform reduced integration (URI) and selective numerical integration (SI) schemes is developed. The approach is simply implemented by a small change of the standard technique and is applicable to arbitrary non-linear constitutive laws including isotropic and anisotropic material behaviors. The implementation of the proposed SI elements is straightforward, while the development of the proposed URI elements requires 'anti-hourglass stresses' which are evaluated by classical constitutive equations. Several numerical examples are given to demonstrate the performance of the suggested formulation, including mechanical problems, heat conduction and thermomechanical problems with emphasis on sheet forming processes.